gistfile1.js

function background(iterator, produceNextValue, callbacks) {
  var value = {}, hasValue = produceNextValue(value);
  callbacks = callbacks || Object.extend([], {
    after: function(callback) {
      this.push(callback);
      return this;
    }
  });
  
  if (hasValue) {
    iterator(value.value);
    background.defer(iterator, produceNextValue);
  } else {
    callbacks.invoke("call");
  }
  
  return callbacks;
}

Object.extend(Array.prototype, {
  backgroundEach: function(iterator) {
    var index = 0;
    return background(iterator, function(memo) {
      if (index == this.length) {
        return false;
      } else {
        memo.value = this[index++];
        return true;
      }
    }.bind(this));
  }
});